1. Field of the Invention
The present invention relates generally to a steam turbine plant and more particularly to steam extraction control for an extraction turbine plant which extracts a portion of main steam from an intermediate stage of a steam turbine and delivers it to a demander.
2. Description of the Related Art
In general, a steam turbine plant extracts a portion of main steam from an intermediate stage of a steam turbine having a plurality of stages and delivers the extracted steam to a demander (e.g., turbine auxiliary machinery such as a feed-water heater and a deaerator, or a warming process in a power plant or various processes in factories attached with a power plant for personal use). Extracting steam from the intermediate stage of a steam turbine as described above means that a steam-reducing action in a steam turbine is used to provide steam with pressure needed by a steam demander.
Such an extraction turbine plant involves a problem with a turbine trip resulting from steam extraction. FIG. 3 represents the relationship between steam extraction structure and stage inlet/outlet steam differential pressure resulting from steam extraction. As seen from the figure, extracting steam from an intermediate stage increases stage inlet/outlet steam differential pressure. In such steam extraction structure, if an extracted steam flow exceeds a given level because of an abnormal increase in demanded amount of extracted steam, a pressure difference is excessively increased between a steam inflow portion and a turbine stage to which an extraction steam supply pipe is connected in a turbine. The stage inlet/outlet steam differential pressure produces stress on a turbine blade in such a manner that the turbine blade is pulled toward the downstream side of the turbine. If the stage inlet/outlet steam differential pressure exceeds the given level, the stress resulting from the stage inlet/outlet steam differential pressure exceeds the design intensity, which probably damages the turbine blade. In order to avoid such damage to the turbine blade, differential pressure between turbine stages has heretofore been monitored by a post-turbine-first-stage pressure gauge and an extracted steam pressure gauge. In general, if the differential pressure exceeds a predefined limit value, then the turbine is tripped.
In an extraction turbine plant used for power generation, shut-down of a steam turbine due to turbine trip causes a large economical loss. Therefore, steam extraction control is needed which can preferably avoid a turbine trip. On the other hand, in the steam extraction control it is important to stably supply extracted steam to a demander. The technique disclosed in JP-A-2000-257405 is known as a technique addressing such problems.
The steam turbine plant operating method disclosed in JP-A-2000-257405 targets the operation of a steam turbine plant which includes a flowmeter which measures extracted steam flow; an extraction steam control valve which controls the extracted steam flow; an extraction steam stop valve which stops extraction of steam; a post-turbine-first-stage pressure gauge which detects steam pressure at the steam inlet portion of the turbine; and an extracted steam pressure gauge which detects extracted steam pressure. In the normal state, the opening of the extraction steam control valve is feedback controlled based on the measurements of the extraction steam flowmeter and the demand plan amount of extracted steam. On the other hand, a difference between the respective measurements of the post-turbine-first-stage pressure gauge and extracted steam pressure gauge are constantly monitored. If the pressure difference exceeds a preset specified value, since the turbine runs into danger, one or both of the extraction steam control valve and extraction steam stop valve are fully closed to stop steam extraction. This can avoid tripping the turbine.
It is to be noted that steam extraction in a steam turbine is disclosed by JP-A-7-180507, JP-A-10-110602, JP-A-7-34809, JP-A-2000-161009 and JP-A-8-312309 as well as by JP-A-2000-257405.